CHEK2 and breast cancer: The CHEK2 1100delC variant has been reported to be a low-penetrance breast cancer susceptibility allele (Vahteristo et al. 2002; CHEK2 Breast Cancer Case–control Consortium CHEK2*1100delC and susceptibility to breast cancer 2004; Oldenburg et al. 2003; Kuusisto Kirsi et al. 2011).